Pyroclastic Density Currents

To PDC or not to PDC? A matter of terminology...

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  1. Why is the difference between pyroclastic density current and pyroclastic flow so important? Pyroclastic flow is a term that has been used for decades; many people will have learned it at school and during geology degrees. But around 10 years or so ago, many volcanologists stopped using this term to describe these rapidly moving mixtures of hot gas and volcanic particles. Why?

    Pyroclastic flow deposit was the word that was used to describe deposits that are massive (i.e. they do not contain any sedimentary structures), poorly sorted and thicken into palaeovalleys. The deposits were interpreted to record a current which had halted as one mass – a frozen current almost. Any changes up through the deposit were thought to record vertical changes within the current. The current itself was thought to have a higher concentration of ash and large volcanic particles than it did gas.

    A second type of deposit was also recognised at explosive volcanoes. This deposit was finer grained than a pyroclastic flow deposit and was bedded, often showing low-angle cross-stratification. Beds are formed layer by layer, so these deposits could not record a frozen current; rather the current formed the deposit through time. These currents were thought to be turbulent and contain a higher concentration of gas than the flows did. Because these deposits were formed in a different way to flow deposits, volcanologists named them surges.

    The terms pyroclastic flows and surges became established and represented two very distinct types of volcanic current. A flow had a high concentration of particles and froze as one mass forming a massive deposit and a surge had a low concentration of particles, was turbulent and was built up layer by layer.

    However, volcanologists began to find examples where massive deposits would change into bedded deposits. Often a massive deposit was found at the bottom of a valley and the bedded deposits high up on valley sides. More and more though, people found examples where one type of deposit would transform into the other along a deposit or up through a deposit vertically. This meant that the deposit could not have been formed by two distinct currents, but rather by one current that changed through time or space. Also, volcanologists began to recognise vague bedding in ‘massive’ deposits, and found evidence that these deposits changed in composition vertically though them. In some cases, the direction of the current recorded in the deposit also changed vertically through it. Researchers realised then that these deposits could not have been formed by freezing a current as one mass and instead it must have been formed layer by layer like a surge deposit.

    These discoveries dramatically changed the way volcanologists interpret the deposits and how they think about the currents that formed them. If a dilute (more gas) current can change into a high concentration current through time and space and deposit both bedded and massive deposits respectively, what name do we use to describe these currents? The term flow and surge are embedded in the literature and in people’s minds to a particular type of deposit and a particular type of mechanism to create that deposit. To attempt to wipe the slate clean of confusing terminology, volcanologists who study these currents coined the term Pyroclastic Density Current. This term just describes a current of gas and volcanic particles. The current can be dilute, turbulent, a gradation of either and change its type through time and space. It can produce deposits that are bedded, massive, poorly sorted, well sorted and a whole spectrum in between. The deposits now have their own terms to describe them so process and deposit now have separate names, breaking the genetic link between them.

    As someone who studies these currents and attempts to interpret their deposits, the distinction is an important one. But not everyone agrees and interesting conversations are to be had, such as the one storified below on Twitter.

  2. A private reply asked "Do journalists get a pass on that? Readers won't go for pyroclastic density current!". My response:

    "I'd argue not. Sorry! It perpetuates incorrect definitions and understanding. It is a mouthful, I know. But flow hasn't been used scientifically since 1992 [or 2002 depending on how into the lit scientists are]. If readers can deal with 'pyroclastic', why not 'density current' too?

  3. So, what changed in 1992?
    Me: Major publication by Branney and Kokelaar followed up by their book in 2002. Paradigm shift in understanding of how currents behave.

    Basically,how we interpret the deposits have dramatically changed.Flow&surge were terms inherently linked with old understanding. New terms had to be used to distinguish between the old and new ways of thinking. Explaining this might be blog worthy! 

  4. OK, you got me. Maybe 'correct' was the wrong terminology to use here...